The FBI late Tuesday arrested a man in connection with the case of Usaama Rahim, who was fatally shot by Boston police earlier in the day following a long-running terror investigation .

Christina DiIorio-Sterling, a spokeswoman for U.S. Attorney Carmen Ortiz, said David Wright was taken into custody from his home in suburban Everett, Mass. She said Wright will face federal charges and is expected to appear in U.S. District Court on Wednesday.

She wouldn't specify the charges, but confirmed they're related to the Rahim investigation.

Rahim was allegedly wielding a military-style knife in a busy parking lot on the city's southwest side. He refused repeated commands to drop the weapon when an FBI agent and a Boston police officer opened fire, Boston police spokesman Officer Stephen McNulty said.

For at least the past 18 months, a senior U.S. official said, Rahim was the subject of a federal investigation into homegrown violent extremists, becoming sympathetic to the cause of the Islamic State or ISIL.

The inquiry was being conducted by the FBI's Joint Terrorism Task Force and was one of a growing number of such cases now underway in every FBI field office in the country.

Rahim is the brother of prominent Boston-area imam Ibrahim Rahim who, in a series of Twitter postings, asked for time to "grieve'' before making formal statements on the incident.

"We are deeply grieved by the loss of Usaama,'' Rahim wrote. "We understand the need 4 info but ask that the press 2 give us time to grieve as a family.''

In one of the postings, the imam said his brother was shot three times in the back by authorities.

The U.S. official, who isn't authorized to speak publicly, disputed that account, saying the suspect was believed to advancing on the authorities when they opened fire, hitting him in the torso.

Muslim advocates issued a statement last night, urging release of any video that may have been taken of the deadly altercation. They also asked for law enforcement transparency.

"We are hearing two different narratives of the incident," said Farhana Khera, executive director of Muslim Advocates. "The victim's family has said that Rahim was shot in the back as he was talking on the phone with his father, while law enforcement officials have claimed he was shot in the front.

Khera added, "During a press conference by the FBI and the Boston Police Department soon after the shooting, officials clearly said that there is no ongoing threat in this case to the American people. Given the lack of a continuing threat, the public deserves to understand why police were spying on and fatally shot Rahim. This transparency is critical to gaining trust of law enforcement by communities that are all too often victims of police surveillance and killings."

Local District Attorney Daniel Conley said the circumstances related to the shooting were under investigation.

The terrorism-related investigation, meanwhile, remained "active and ongoing,'' McNulty said.